Manufacture of aromatic hydrocarbons



Patented May 23, 1961 MANUFACTURE OF AROMATIC HY DROCARBONS Otto Probst and Ernst Hieronymus, Frankfurt am Main,

Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed Feb. 13, 1959, Ser. No. 792,949

Claims priority, application Germany Feb. 18, 1958 7 Claims. (Cl. 260-6735) The present invention relates to a process for the manufacture of aromatic hydrocarbons.

It is known that aromatic hydrocarbons can be obtained from diisobutylene, triisobutylene, their hydrogenation products or mixtures of these substances by passing them in vapor form in the presence or absence of a carrier gas at a raised temperature over a known aromatization catalyst.

Although the dilution with the carrier gas involves already a considerably improved yield of aromatic hydrocarbons, the hydrocarbon that has not been converted into aromatic hydrocarbon is partially split into gaseous hydrocarbons.

We have now found that the unreacted aliphatic hydrocarbons can substantially be prevented from being split into undesired by-products, while the rate of conversion remains practically the same, when the aromatization is efiected under reduced pressure. This fact is especially surprising since according to the Le Chatelier-Braum principle low molecular cleavage products should have been expected to form in an increased proportion. The reconduction of an increased amount of unreacted aliphatic hydrocarbons involves yields which are considerably higher than those obtained at atmospheric pressure or under a raised pressure with or without a carrier gas. The conversion of the aliphatic hydrocarbons to the de sired aromatic hydrocarbons is not impaired.

The process of this invention oifers the advantage that aromatic hydrocarbons are obtained in better yields than when working at atmospheric pressure. As compared with the process wherein the hydrocarbons to undergo aromatization arediluted with other gases, the present invention offers the further advantage that the aromatic hydrocarbons are considerably simpler to work up with less expense of energy in view of the fact that the starting hydrocarbons need not be diluted withstrong diluents which are otherwise necessary to obtain a high rate of conversion. The higher boiling conversion products can readily be separated by condensation on the suction side of the vacuum pump. The lower boiling products are obtained with high tension on the pressure side of the pump so that they can be liquefied while cooling moderately. The application of reduced pressure may, of course, be combined with the use of diluents, especially when the hydrocarbons are only little diluted.

In accordance with this invention aromatic hydrocarbons, more especially para-xylene, are produced as follows from diisobutylene [2,4,4-trimethylpentene-(1) or 2,4,4-trimethylpentene-(2) ],triisobutylene, their hydrogenation products or a mixture of these hydrocarbons. The hydrocarbons used as starting material are aromatized in vapor form under reduced pressure and at a raised temperature, advantageously between 450 C. and 650 C., preferably 480 C. and 590 C., in the presence of a known aromatization catalyst and, if desired, in the presence of a diluent.

The increase in the yield of aromatic hydrocarbons is already observed at a slightly reduced pressure; this increase in the yield is, however, especially pronounced when the reaction is carried out under a pressure of below 0.5 atmosphere absolute. It is advantageous to carry out the reaction under apressure of 50-150 mm. of mercury, which can be produced without difliculty.

The considerably improved yield of aromatic hydrocarbons obtained by the process of this invention is independent from the space/time load. It is, however, especially advantageous to use a space/time load of 0.4 to 0.9 liter of liquid hydro'carbon per liter of catalyst volume and per hour.

When it is desired to additionally dilute the starting material, it is advantageous to use as diluent especially nitrogen and/ or methane and/or an aromatic hydrocarbon, such as benzene and/or its homologs, for example toluene, xylene or ethyl benzene. In view of the fact that hydrogen is evolved during the reaction, it is in most cases not advisable to additionally admix the starting material with hydrogen and/ or a substance splitting oif hydrogen. It is also suitable to remove all substances which may impair the aromatization, such as carbon dioxide or steam. The aromatization should advantageously be etfected with the exclusion of moisture, and the vaporous starting mixture used should contain less than 0.05% by volume of steam.

As catalysts there may be used the known aromatization catalysts, advantageously those containing oxides of metals of subgroup 6 of the periodic table, such as chromium oxide, molybdenum o'xide and/ or tungsten oxide, preferably in a proportion of at least 1%. [It is especially advantageous to use a chromium oxide/aluminum oxide catalyst, or more especially a catalyst containing chromium oxide/potassium oxide/ cerium oxide/ aluminum oxide, in which the relative proportion of the individual components varies within the limits of 540%, l10%, 0.5-

5% and 93.545%. The potassium oxide, chromium 0x ide and cerium oxide may also be present in an amount greater or smaller than indicated above; the cerium oxide may even be omitted. There may also be employed catalysts which consist of chromium oxide/aluminum oxide and platinum or palladium additions, for example the catalysts described in French Patent 1,106,986.

The examples below indicate that a considerable portion of unreacted not aromatized starting material istrecovered unchanged which may be used. again atter separation- Contrary thereto, when the process is carried' out at atmospheric pressure and the starting material is diluted with hydrogen, a considerable portion of unreacted diisog butylene is split into gaseous hydrocarbons.

The following examples serve to illustrate the inven-' tionjbut they are not intended to it theretoz Tr T Example 1 'yAl O was impregnated with chromic acid, potassium nitrate and cerium nitrate, calcined at 550 C. and reduced in a current of hydrogen to produce a catalyst which contained 12.1% of Cr O 1.7% of K 0 and 1.5% of Ce O Over '120 cc. of the catalyst so prepared were passed 121.8 grams of diisobutylene within 3 hours at 500 C. and under a pressure of 300 mm. of mercury, measured at the inlet opening of the reaction tube. The diisobutylene had previously been evaporated and heated to 5 00 C. The amount of hydrocarbon introduced corresponded to a space/ time load of 0.47 liter of liquid hydrocarbon per liter of contact space and per hour.

73 grams of liquid reaction product were obtained corresponding to 60% by weight, calculated on the starting material used. The liquid reaction product contained 51% by weight of paraxylene and 42% by weight of unreacted diisobutylene. The yield of paraxylene obtained in .3 a single passage corresponded to 30.6% by weight, calculated upon the starting material; the yield calculated upon reacted diisobutylene amounted to 43% of the theory.

When the reaction is carried out at atmospheric pressure under otherwise identical conditions and when the evaporated diisobutylene is diluted with 60% by volume of hydrogen, the yield obtained in a single passage is only 11.5% by weight, calculated on the starting material used, or only 33.8% of the theory, calculated upon reacted diisobutylene.

Example 2 The oxide catalyst used contained 'y-aluminum oxide as the carrier, 16.6% Cr and 3.0% K. Over 1.27 liters of this catalyst were passed 576 grams/hour of diisobutylene in vapor form at 520 C. and under a pressure of 130 mm. of mercury at the inlet opening of the furnace. The diisobutylenehad been pre-dried to contain in vapor form less than 0.05% by volume of steam. 412 grams of liquid product corresponding to 71.5% of the starting material used were obtained in the course of 1 hour. The liquid product contained 39.2% by weight of para-xylene and 54.8% of unreacted diisobutylene. This corresponded to a yield of 28% by weight in a single passage calculated on the starting material used. The yield, calculated on unreacted diisobutylene, was 49% of the theory.

Example 3 V Example 4 Over 1.27 liters of the same catalyst as used in Example 2 were passed 630 grams/hour of 2,2,4-trin"1ethy1pentenev in vapor form at 570 C. and under a pressure of 150 mm. of mercury at the inlet opening of the furnace. 348 grams of liquid reaction product were obtained in the course of 1 hour, which contained para-xylene in a concentration of 36.5% by weight, corresponding to a yield of 20.2% by weight of para-xylene in a single passage.

Example 5 Over 1.27 liters of the same catalyst as described in Example 2 were passed 603 grams/hour of triisobutylene in vapor form at 550 C; and under a pressure of 160 mm. of mercury at the inlet opening of the furnace. 43.1% by weight of liquidreaction product, calculated upon/the starting material, were obtained, which contained para-xylene in a concentration of 54.4% by weight.

Para-xylene was obtained in a yield of 23.4% by weight calculated on the starting material used.

We claim:

1. In a method of aromatizing an aliphatic hydrocarbon selected from the group consisting of diisobutylene, triisobutylene, hydrogenation products thereof and mixtures thereof by passing vapors of said hydrocarbons over 7 an aromatization catalyst at an elevated temperature, the

improvement which comprises carrying out the aromatization under a pressure lower than 0.5 atmosphere.

'2. Method as defined in claim l, wherein a mixture of a hydrocarbon is aromatized in the presence of a gaseous carrier.

3. Method as defined in claim 1, wherein a catalyst is used which contains from 5 to 40 percent of chromium oxide, 1 to 10 percent of potassium oxide, 0 to 5 percent of cerium oxide, and 94 to percent of aluminum oxide.

4. Method as defined in claim 1, wherein the aliphatic hydrocarbon is diisobutylene.

5. In a method of aromatizing an aliphatic hydrocarbon selected from the group consisting of diisobutylene, triisobutylene, hydrogenation products thereof and mixtures thereof by passing vapors of said hydrocarbons over an aromatization catalyst at an elevated temperature, the improvement which comprises carrying out the aromatization under a pressure lower than 0.5 atmosphere at a space time throughput of 0.4 to 0.9 liter of the hydrocarbon to be aromatized, calculated as liquid hydrocarbon, per liter of contact space.

6. In a method of aromatizing an aliphatic hydrocarbon selected from the group consisting of diisobutylene, triisobutylene, hydrogenation products thereof and mixtures thereof by passing vapors of said hydrocarbons over an aromatization catalyst at an elevated temperature, the improvement which comprises dehydrating the initial hydrocarbon to a water vapor content of less than 0.05% by volume and'carrying out the aromatization under a pressure lower than 0.5 atmosphere.

7. In a method of aromatizing an aliphatic hydrocarbon selected. from the group consisting of diisobutylene, triisobutylene, hydrogenation products thereof and mixtures thereof by passing vapors of said hydrocarbons over an aromatization catalyst at an elevated temperature, the improvement which comprises carrying out the aromatization under a pressure in the range from 50 to mms. of mercury.

References Cited in the file of this patent UNITED STATES PATENTS 2,754,345 Kirshenbaum July 10, 1956 2,857,442 Hay Oct. 21, 1958 2,880,249 Raley et al. Mar. 31, 1959 

1. IN A METHOD OF AROMATIZING AN ALIPHATIC HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF DIISOBUTYLENE TRIISOBUTYLENE, HYDROGENATIOIN PRODUCTS THEREOF AND MIXTURES THEREOF BY PASSING VAPORS OF SAID HYDROCARBONS OVER AN AROMATIZATION CATALYST AT AN ELEVATED TEMPERATURE, THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE AROMATIZATION UNDER A PRESSURE LOWER THAN 0.5 ATMOSPHERE. 